Conventionally, there has been known a semiconductor device which has a breakdown strength structure in a gate pad portion (see patent literature 1, for example).
The conventional semiconductor device 900 includes, as shown in FIG. 22 which is a plan view of the semiconductor device 900, an element portion 970 and a gate pad portion 980 on the same semiconductor substrate 910. The semiconductor substrate 910 is made of silicon, for example. A breakdown strength of the semiconductor device 900 is 60 to 300V.
As shown in FIG. 23 which is a cross-sectional view, the element portion 970 includes: an n-type low resistance semiconductor layer 912; an n-type drift layer 914 positioned on the low resistance semiconductor layer 912; a p-type body layer 916 positioned on the drift layer 914; a gate trench 918 formed by opening the body layer 916 and reaching the drift layer 914; an n-type source region 920 arranged in the inside of the body layer 916 and formed such that at least a portion of the n-type source region 920 is exposed to an inner peripheral surface of the gate trench 918; a gate insulation layer 922 formed on an inner peripheral surface of the gate trench 918; a gate electrode layer 924 formed inside the gate trench 918 by way of the gate insulation layer 922; and a source electrode layer 928 formed in an insulated manner from the gate electrode layer 924 and being brought into contact with the source region 920. Further, the element portion 970 includes: a source trench 960 formed in the body layer 916 and the drift layer 914 such that the source trench 960 reaches the low resistance semiconductor layer 912; an insulation layer 962 formed on an inner peripheral surface of the source trench 960; and a polysilicon layer 964 formed inside the source trench 960 by way of the insulation layer 962 and being connected to the source electrode layer 928.
A cross-sectional view of the gate pad portion 980 is shown in FIG. 24. Although most of constitutional elements of the gate pad portion 980 are substantially equal to the constitutional elements of the element portion 970 shown in FIG. 22, a gate electrode line 966 is provided in place of the source electrode layer 928, a gate trench 918 has a larger width than that of the element portion 970, and a gate electrode layer 924 is connected to the gate electrode line 966. On the other hand, a source region 920 and a polysilicon layer 964 are electrically insulated from the gate electrode line 966 by a field oxide film 968. In an actual semiconductor device, the gate pad portion is connected to the outside of the device by a wire bonding or the like and hence, the gate pad portion is required to have a size of at least approximately several hundred microns.
According to the conventional semiconductor device 900, in a turn-off state, a depletion layer extending to the drift layer 914 not only extends toward the low resistance semiconductor layer 912 from the body layer 916 but also extends in a narrowing manner between neighboring source trenches 960 since a potential of the polysilicon layer 964 which faces the insulation layer 962 in an opposed manner is held at a source potential. Due to such an action, the concentration of the drift layer 914 can be increased compared to a prior art even at the same breakdown strength and hence, resistance in an ON state (ON resistance) can be reduced.
On the other hand, as a method of reducing ON resistance while acquiring a high breakdown strength, it is possible to use a wide gap semiconductor such as silicon carbide in place of silicon as a semiconductor material.